Selective signaling circuits



Feb. 1, 1927.f

R. H. MLLs SELECTIVE S IGNALING CIRCUITS Filed Sept. 29, 1924 2 Sheets-Sheet l Feb. 1 1927. 1,616,193

R. H. MILLS SELECTIVE SIGNALI NG CIRCUITS energica ret. i, i927.

imi-TED sIArEs ,f anais-3- l arent citric@ RALPH n. MILLS, oEEAsT'oRANGE, NEW JERSEY, AssreNoR To WESTERNV ELECTRIC COMPANY, INcoEroRATED, or NEW YORK, N. Y., A CORPORATION or NEW YORK.

. I,ESIVEIIJEL'IT117131 SIGNALING CIRCl'll'liS Application led September 29, 1924. Serial No.-'?40,528. i

'lihis invention relates to selective signaling circuits such as are used, for example,

toassocatethe transmitting and receiving branches of a two-way multiplex .carrier .5 Wave signaling system with a common transmission medium.- i An object of the invention is to improve ,the transmission of such a selective signaling circuit. s

SeveralJ types of carrier .Wave signaling systems to which the invention may he aplied are described in an article by Messrs. olpitts and Blackwell, entitled Carrier current telephony and .telegraphy published in the Transactions of the American institute of Electrical Engineers, Volume XL,

1n the varrangements illustrated in Figs. 42 and 49 of that article the several trans- 20 `mitting .band filters in a given system have their end sections all `connected in series with each other Vand the common transmission circuit. Likewise, the several receiving band filters have their end sections connected ink series with each other and with the common receiving cfrcuit. The common transmitting and common receiving circuits are then connected to' a common transmission line by means of a hybrid coil and balancing network. f

signaling systems all of the band filters, both transmitting and receiving, havetheir end sections connectedin series with each other and with the common transmission line. 1n

other 'types of systems the various band filters are connect-ed in parallel with each other `and the transmission line. l

In all these cases, Where a plurality of 'band filters are connected either in parallel o1' in series to the common transmission line, it is customary to terminate the' filters at the ends where they are joined together in a special fractional termination to iinprove the transmission loss characteristics of the lilters in their transmitting regions at and in the vicinity of their respective cut-ofi' frequencies. Where the filters transmit successive bands of frequencies the impedances of the respective filters when thus terminated 4improve the transmisson characteristics at the adjacent side of the next filter on either side. r1`he use of such fractional terminations is disclosed in a patent lrier waves of` different frequencies. Low

of'U. J. Zohel, No. 1,557,230, @ctober 13, 1925.

Even when ,this type of fractional termination is employed, however, it is found that there is an uncompensated loss in the end filters `of a given lgroup at those frequencies which are not adjacent inthe frequency scale tothe hand transmitted by another one ofthe filters. Such an uncompensated loss occurs, for example, at and v in the vicinity of the lowest cut-off frequency of the Iowest of a group of several band tilt-ers andsimilarly at, and in -the vicinity of the highest cut-off frequency of the highest of such a group of filters.

According to a feature of the invention the transmission characteristics of the loW- est and highest of a group of filters operating 4either in parallel or in series are im-` proved by means of lsimulating networks. which have the same react-ance as an additional adjacent `filter would have 'at the desired cut-ofil frequency. v

The invention will be described as applied to a multiplex carrier current telephone ,sys-v tem, but it will he understood that it may also be applied to other signaling systems. f

ln the drawings :f

i, Fig. l is a diagrammatic illustration of one terminal of a multiplex carrier current ln other known types of carrier Wave' telephone system embodying the invention.

Fig. 2 shows curves illustrating the transmission loss characteristicsof certain o f the terminal filters of F ig. 1.

Fig. 3 showscurvesillustrating theI impedance characteristics of these filters. F ig. 4 shows a modification of the system of Fig. l.

The carrier terminalapparatus shown in Fig. l comprises a plurality of two-way carrier Wave signaling channels I', II and III, each having a1 transmitting and a receiving branch. 4

FElie transmitting branches of these chan'- nels are connected in parallel to the transf mission line ML through a common transniitting circuit TL, and the receiving branches vare connected'in parallel to the iie ML through a common receiving circuit Transmissions in the several two-way channels I, l1 and H1 are eected by cari and .higlrpassv filters LP and HP are asso-v ciated with the common transmitting and receiving circuits TL and RL, respectively;

and serve to separate the directional groups of carrier waves to the respective branches of the several channels. These filters may be of the type diselosed'in the United States patent to G. A. Campbell, No. 1,227,113, issued May 22, 1917.

The eorrespondin elements in the several two-way channes are much alike, differing only to the extent necessary to accommodate the different yfrequency carrier waves employed. For this reason only the elements of channel I will be described i'n detail. A

Corresponding elements in" the several channels are designated by the same reference letter, the channel being designated by the numeral following the letter. Certain of the elements of channel III have been omitted from the drawing forl the sake of clearness. y

The two-way carrier wave signaling channel I has in its transmitting branch a transmitting oscillator T01, a modulator M1 and a transmitting band filter vTBFl. The receiving branch of the' carrier wave signaling channel I includes a receiving band filter RBF1, a detector and amplifier DA1, a receiving oscillator B01 and a voice frequency filter F1.

A low frequency line L1, whichmay be an ordinary subscribers telephone circuit, is

conjugately connected to the transmitting and receiving branches of channel I by means of a hybrid coil H1 and a balancing networkl N1. This conjugate connection enables independent transmission to be carried on in t both directions between the line L1 and the high frequency terminal apparatus. Oscillators T01 and R01 modulator M1, and the detector and ampli er DA1 -may be of any suitable design, such as the well known vacuum tube type. Devicessuitable for this purpose are described in the Colpitts and Blackwell article, supra.

The voice frequenc filter F1 maybe of the ordinary low pass lter type disclosed in the Campbell patent, supra, and is designed to transmit to the low frequency line L1 currents of voice frequencies appearing in the output. of the demodulator and'amplifier DA1 and to suppress from transmission the higher frequency components of demodulation.

The transmitting band filter TBF1 and the receiving band filter BBF1 may be of the composite wave filter type discussed in part 3. of an article on the Theory and design of uniform and composite electric wave-)- filters, by O. J. Zobel in the Bell System Technical Journal of January, 1923. i These filters consist, in general, of a plurality of sections having'series and shunt reactances designed, according to well known laws, to

transmit with substantially negligible atten-- nation sinusoidal currents of all frequencies` lying between two selected limiting fre'- quencies, while attenuating and approximately extinguishing currents of neighboring frequencies lying outside of the said limiting frequencies. `Filters of this type are known as suppression filters and are particularly advantageous where a sharp cutoff between frequencies in the transmitted and suppresed ranges is desired.

` The carrier system outlined above is of the type in which the carrier wave of each channel is suppressed from transmission -when no signals are being transmitted, and

apparatus at the distant terminal. A. sys! tem of this general character is described in connection with Fig. 49 of the Colpitts and Blackwell article, supra.

Transmitting band filters TBF1, TBF2 and TBF3 are designed to transmit respective bands of frequencies comprised within the lowerl group in an ascending frequency scale. The frequencies comprised within this lower group are transmitted to the line ML through the low pass filter LP. The receiving band filters RBF1, .RBFl and HBF3 are designed to transmit respective bands of frequencies comprised within the upper group in the frequency scale which are transmitted thereto by the high pass filter Hl), a certain particular band of these frequencies being diverted into filter RBF1, another particular band into RBF2 and another into RBF3. These three bands of frequencies originated at the other or distant end of the line ML. l

When the transmitting and receiving band 'filters of channels I, II and III arel connected in parallel to the transmission line ML as shown in Fig. 1,-or in series as shown in F ig. 1, or in series as shown in Fig. 4, it is well known that the transmission of each filter isI affected by the impedance of the other filters Which are connectedto the same common transmission circuit. IVhen the band filters are connected to the transmission line in parallel as shown in Fig. 1'

they are preferably terminated at theend nearest the line ML in fractional series arms, instead of mid-series arms, in the manner disclosed in the Zobel application, hereinbefore mentioned. '-As described in the Zobel application, the fractional terininations are' so proportioned that for the free transmitting range of each filter the remaining filter or lters operate to reduce impedance lirregularities of 4each filter.

A particular advantage of this arrangement is that the reactance annulling effect` of each filter on either side of a given filter in the' frequency .displacement scale tends to improve the transmission loss characteristic of the given lter at and in the vicinity amp1e,.sinc`e transmitting band filters TBFl, TBFZ'and TBF3 are all connected in parallel to the common transmitting circuit TL, 'the transmission loss characteristic of filter TBF? is improved in the neighborhood of its lower cut-off frequency by the presence of filter TBFl and near its'upper cut-off frequency bythe presence of filter TBFS.

This is shown graphically by the filter ycurves ofL Fig. 2, wherein the transmission `loss of the various filtersTBF1, TBFZand TBF is plotted against' frequency; The transmission characteristic' of filter TBF2 is improved at and in the neighborhood of its lower cut-off frequency fzl bythe pres- TBFl islikewise improved at its upper cutoff frequency f1? yby the'presence of filter TBF2, but since no filter exists in the fre-v quency scale `below filter TBF1 the'tra-nsmission losses in the neighborhood ofthe lower cut-off frequency fll are-practically the same as they would be if yfilter 4TBF1 were an isolated filter, as shownl by the round solid line portion of the characteristic curve of this filter at frequency ff. A similar con# dition exists in the neighborhood of the upper cut-ofi` frequency ff of filter TBFB.

ere, since no filter 1s present in thefrequency scale above'filter TBF3, the transmission losses of this filter in the neighborhood of its upper cut-off.frequencyfl are substantiallyy the same as if filter TBFa v were an isolated filter as alsoshown by the rounded solid portion of the characteristic l curve of this filter. p

Iffthe group of band-filters were extended below the filter TBF1 in the frequency scale, `then suchA an added filter A adjacent the filter TBFl. and having' the dotted line translnission loss characteristic shown in Fig. 2, would have the effect of improving the transmission characteristic] of the filter TBF1 at its lower cut-off frequency fll. In

a similar manner if the group of filters were extended above filter TBF3 in the frequency scale, then the presence of such an added .filter vB adjacent to the filter TBF3 would have the effect of improving thetransmis- .sion characteristic of filter TBF3 at its This improve-y ment is 'indicated by the less rounded dotted upper cut-off frequency faz'.

line portion of thev characteristic curves at l U 'ff and at f 2.

of the adJacent cut-ofi` frequency. For eX- In much tahe same way that the transmission loss of any band filter is improved near its upper cut-offl frequency or near its lower cut-off frequency due to the presence ofthe impedance which is offered by another filter which is passing currents in the frequency f scale'above this upper cut-,offor below this lower cut-ofi', applicant has discovered that the transmission loss characteristic of `filter TBF1 at its lowercut-of frequency fil may be improved in accordance with this invention by connecting in parallel therewith a simple impedance network which simulates the impedance of the fictitious filter A. In a similar manner the transmission loss characteristicA of the end filter TBF3 is improved at its upper cut-ofi'l frequency ff bymeans of. a simple impedance network which simulates the impedance of the fictitious filter B.

Fig. 3A illustrates the impedance characteristicsof band filters TBF` TBF2 and- TBFS, as well as the supposititious impedance characteristics of the fictitious band filters A and B. The impedance characteristics ,of the fictitious' band filters A and B indicated by the dotted curves of Fig.l 3 may, o course, be readilydetermined since these filters would be designed to transmit a Well-defined range of frequencies if they were' actually included in the frequency l scale.

TBF? and Tl3F"'comp1ise`s afpu're resistance anda reactance. Tle resistance of these filters over vthe Arespective transmission The impedance of each of the filters TBFl,

y lOl:

110. "ranges is indicated by the,` characteristic L I curves R1'9 Rt and R, while-the respective positive reactances are indicatedY by'curves i-m1, +1172 and +iva and the respective negative reactances *are designated by curves -w1, m2 and w3. The resistance of the fict1t1ous filters A andl over their respective transmlssion ranges 1s indicated by the dotted curves Ra and Rb. The positive and negative reactances of the fictitious filtenAA are indicated bythe dotted curves -l-.ra and -a", while the positive and negati-ve reactances of filter B are indicated by the dotted characteristics +112 and -mb. v

ln order to improve the transmission loss of the' ba'nd filter TBF1 an impedance simi- .larto that of the fictitious filter A may be inserted Ain parallel therewith. However..

since in the present case it is only desired' to improve the transmission loss of filter TBF1 in theneighborhood of its lower cutoff frequency fil, the network which is to\ be inserted in parallel with the filter TBI"1 will only be required vto simulate thei'mpedancek of the `fictitious filter A` through a small range of transmission. The :impedance of the fictitious filter A at the frequency ff comprises .simply the' positive reactance -l-a, as indicated by the dotted ordinate of Fig. 3. In its simplest case where filters TBF1, TBF2 and TBF3 pass bands of frequencies in an ascending,lr scale, the reactance +008 may be vonlyanl inductance coil since an inductance coil has a positive reactance.

In this case, Las well as when the limpedance of the fictitious filter is simpl a ynegative reactance, `the value of the simu ating network may be computed from the J formulae given below provided the other elements in circuit with the filterwhose cut-off 1s to bev improved` are fixed impedances.

This, in fact, is the general case, but where impedance irreglarites exist at certain frequencles, forexample, 1n the modulator M1 in circuit with band filter TBF1 it is `.obvious that the impedance of this filter at the terminal adjacent the common transmitting circuit willV differ at certain frequencies from the com uted impedance re re- .sented by the impe ance curve of this lter in Fig. 3. vWhen such inpedance irregularities exist it is preferableto perfect the simulation network by trial, measuring theI impedance of the circuit by means of any suitablemeasuring apparatus.

Where the elements in circuit with the lfilter TBF.1 are fixed impedances the value of the coil simulating the positive reactance ofthe fictitious filter A maybe obtained 'from' the following formula: wiQvrfL,

where m isvthe reactance that the fictitious filter 'Aavould have at frequency f and L is the resulting inductance of the coil in henries. By substituting specific values of .m and f in this formula 1t will be seen that the inductance of the coil required to approximately simulate the impedance of the fictitious filter A at the frequency fl may be mission loss characteristic of filter TBF1 is improved in the neighborhood of its lower l cut-off frequency fll. The addition of this simulating network has the effect of sharpenin the transmission loss characteristic of the lter TBF1 inthe manner indicated by the dotted portion of the characteristic L curve of this filter shown in Fig. 2.

The transmission lcharacteristic of the filter 'IBF3 may likewise be improved 1n ing band filters work SN3, designed to improve the neighborhood of its upper cut-ofi' frequency ff by inserting in parallel therewith a network which simulates at this particular frequency the impedance of the fictitious filter B whose Vtransmission band lies in the frequency scale just above that of filter TBF3. At the frequency faz the impedance of the fictitious lter B is simply a ne ative reactance as indicated by the otted ordinate of Fig. 3. Since the reactance -b has a negative value it may be represented in its simplest case b a condenser, the capacit of which may e determined from the fol owing formula:

v 21rf6 where w is the known reactance of fictitious filter B at any frequency f and c is the resulting capacity of the condenser in farads.

By substituting in this formula it will b e seen that the capacity of the condenser required to' approximately simulate the impedance. of the fictitious filter B at the.I

frequency f3 may be represented as:

y 2.fsz-xb Thereforevby inserting in/parallel with the filter a" network such as the netwo-rk SN2 lowering a capacity Gthe transmission loss characteristic of filter TBF3 is improved in the neighborhood of its upper Vcut-ofi frequenc 72,2 as indicated by the dotted yportion of t e characteristic curve of thisfilter in Fig. 2'. While the transmitting band filters shown in Fig. 1 have been selected -or the purpose of illustrating the invention, it is to be understood that exactly the same formulae and procedure are to be used in designing simulating networks to improve the transmission characteristic of the receiving band filter RBF1 at its lower cut-off frequency and of the receiving band filter RBF3 at its upper cut-off fre uency. As noted above, these filters are esigned to transmit predetermined bands of frequencies in an ascending frequency scale, and accordingly the curvesl of Figs. v2 and 3 which have been used in connection with the description of filters TBF1, TBFs and TBFS, also apply in every particular to the receiv- RBF1, BBF2 and RBFG. In this case, ofcourse, the simulating netthe transmission characteristic of filter R-BF1 at its lower cnt-off and the simulating network SN4, designed to improve the transmission frequencies and impedanf'es -of the trans mining bandl filters.

l Fior. 4 shows a modification of the inven? tion 1n which the various transmitting and receiving band filters are connected in series with the transmission line instead of in parallel as has been assumed heretofore. lin this case, the transmission characteristics of transmitting band. filters TBF1 and TBF characteristics of filters TBF1 and TBF3 are indicated schematically at TFN and thetwo networks designed to improve the transmislsion characteristics of filters RBF1 and HBFa are indicated schematically at RFN.. .j The impedance characteristics of a group of band filters connected in series is; however,

somewhat different from the characteristics of the group of band filters in Fig. l which are connected in parallel.A For example, the

.' impedance of a fictitious filter A at the lower cut-ofi' frequency .fx1 ofthe .filter TBFl of Fig- 4 instead/of being a positive reac# tance is a negative reactance and'accordlngly in its simplest case the network simulating such a reactance would be a condenser instead of an inductance coil. the impedance of a fictitious filter B at the cutoff frequency faz of the filter TBF3 of Fig. 4 is a positive reactance instead 'of a negative reactancc, and accordingly in its simplest case theI network simulating this reactance would be an inductance coil. The same conditions also ap ly to the receiving band filters RBF1 and BF3 of Fig. 4.

l/Vhile the simulating networks SNl, SNZ, SN3 and SN4 as pointed out aboveare designed to improve the transmission characteristics of the lowest and highest of the transmitting and receiving band filters atV their respective lower and upper cut-off frequencies, it will be understood that thesenetworks maybe designed to improve the transmission characteristics of vthese filters at other points in the transmission range, or in fact throughout the entire transmission range of the respective filters. y

Where a group ofv band filters such as those` herein shown and described are connected either in parallel or in series to a transmission line not only does each adjacent lter have the effect of reducing the transmission loss of any filter in the neighborhoodof the cut-off frequency, but they also increase the loss in the at-tenuating regions of the given filter where, of course,

it is usually desirable to have as large a loss \as possible. This beneficial effect can beproduced by simulating networks which are inserted adjacent to the end filters of the grou to the same degree as it would be by add1- tional adjacent filters.

Similarly, l

The invention 'ma' be employed to adters TBF1 and TBF.3 in the usual manner.

and if filter TBF2 were simply extracted without replacing itlin any Way, it is `ob vious that filter TBF1 would not operate so efficiently atits upper cut-off or filter TBFa at its lower cut-ofi'. Therefore, to reserve'l i the eflicient operation of filters T F1' and,

TBF3 a simulating network may be substituted for filter TBFQ. It would be'nfecessary, of course, thatthis network simulate approximately the impedance of filter TBF2. A similar procedure could be followed if it were desired to eliminate either filter TBF1 or filter TBFf, or both of them.

The invention has been described mainly in connection with its application to the operation of band pass filters because the use of such filters involves a transmission improvement in two regions,.o11e just above the lower cut-offend the other ]ust below the upper cut-off. The invention, however, is equally applicable to the operatlon of low and high pass filters in parallel or in series. .In the case'of filters of this type the im'- provement in the transmission range of the filters would be just, below the lower .cut-off and just abovethe upper cut-off frequencies. Another example of the use of filter.l impedance simulating networks is in the operation of directional or grouping filters at repeater stations and at the terminal stations of carrier current si aling systems. Here low. and high pass directional filters, such as'the filters LP and HP of Fig. 1 are used in parallel, transmissionoccurring through one filter and reception occurring through the other, 'each filter being aided in its transmission near' itscut-off byv the presence of the other. If for 'some reason it is desired to remove from operation one of these parallel filters without impairing the transmission of the other, this might be done by substituting` an impedance network which simulates the impedance of the filter which is withdrawn.

loo

The invention is also susceptible of various other modifications and adaptations,

1. A selective system comprising a common circuit, a plurality of filters with'respective mutually exclusive transmission ranges, said filters including terminal sec.

tions all of which. are connected to said `common circuit, and corrective ymeans also connected to said cgmmon circuit and simuy lating the effect on. the transmission low characteristics ofl the filters transmitting the lowest and highest bands offr'equencies which would be produced Aby other filters of respectively lower and higher frequency ranges.

2. VA selective circuit comprising a plurality of filters with respective mutually exclusive transmission ranges, and corrective means to reduce the transmission loss 'of the filters transmitting the lowest and highest bands of frequencies in the neighM borhood of their respective lower and upper -Y ing their terminals connected to each other and corrective means comprising reactive elements connected across said filter terminals to reduce the transmission loss in the neighborhood of the lower cut-off of the filter transmitting the lowest band of frequencies and in the neighborhood of` the up'- per cut-offof the filter transmitting* the highestband of frequencies.

5. A selective circuit comprising a transmission line, a plurality of filters with respectivey mutually exclusive transmission ranges connectedto said line, and impedance corrective networks connected to said line to reduce the transmission loss inthe neighborhood of the lower cut-off of the filter transmitting the lowest band of frequencies and in the neighborhoodof the upper cutoff of the filter transmitting the highest band of frequencies. y

6. A selectivey circuit comprising a transi mission line, a plurality of lters -each adapted to transmit a bandof frequencies,

lying within` the suppression range of all of the other filters, said filters being connected to-said line'lin such a way that any given filter acts as an -impedance corrective network for the adjacent filter in the neighborhood of the cut-0E frequency nearest the transmission band of the given filter,

and impedance elements connected to`said line to reduce the transmission loss of theY filters transmitting the lowest and highest frequency bandsin the neighborhood of the respective lower and upper cut-off` frequencies of thesefilters.

7. A selective circuit comprising .a trans- V,mission lline, aplurality of l band filters joined at one end to ysaid line so that certain frequencies may be diverted into. one filter and other frequencies into ,another filter,

and a pair of impedance networks connected to said' line, one of said networks-simulating the impedance of a fictitious filter adjacent the lowest filter in the frequency scale,

. `and the other-of said networks simulating the impedancen of a fictitious filter adjacent the highest filter inthe frequency scale.

8. A selective circuit comprisinga transmission line, a plurality vof band filters joined at one end to said line so that certain frequencies may be diverted into one filter and otherl frequenciesjinto another filter, and a-pair of impedance networks connected to said line, one of""said networks simulating the impedance of a fictitious filter adjacent t'he'lowest filter in the lfrequency scale at the lower cut-off thereof, and the other of said networks simulating the impedance of a fictitious filter adjacent the highest filter in the scale at the upper cut-olf thereof.

9. A selective circuit comprising a transmission line, a plurality of filters with re-i spective mutually exclusive transmission ranges connected in parallel tosa'id line,y

and impedancey corrective networks connected in parallel to lsaid line to reduce the transmissionlloss in the neighborhood of the lower cut-off of the filter transmitting the lowest band of frequencies and in the neighborhood of theg upper cut-off of the filter transmitting quencies. j l

10. A selective circuit comprising a trans-- mission line, a plurality of band filters conthe highest band of frenected in parallel to said line so that certain frequencies may be diverted into one filter and other frequenciesinto another filter, and a pair of impedance networks connected in parallel to said line, onek of said networks simulating the impedance of a ctitiousfilter adjacent thelowest filter in, the frequency scale, and the otheriof s aid networks simulating the. impedance 'of a fictitious filter adjacent thehighest filter in 4the/frequency Scale. Y l

In witness whereof,I hereunto subscribe my name this `25th day of September, A. D.,

RALPH H. MILLS. 

